Gas turbine engines are widely applied machines for generating power or thrust. Most typically, they are employed on modern aircraft to provide the propulsion necessary for flight. They may also be used onboard such aircraft for power generation in an APU (Auxiliary Power Unit) capacity to provide for onboard heating, cooling, and ventilation, as well as operational power and lighting systems onboard the aircraft within the cockpit, passenger cabin, and the like. They may also be used in land based applications for generation of electrical power or mechanical horsepower in myriad vehicles and pieces of machinery.
In a typical gas turbine engine, three main sections are provided, namely, a compressor section, a combustion section, and a turbine section. Within the compressor section, ambient air is ingested, highly compressed, and directed through a downstream diffuser into the combustion section. Within the combustion section, the highly compressed air is mixed with fuel within an annular combustion chamber and burned at extremely high temperatures, generating massive levels of heat energy. Moreover, as opposed to internal combustion engines, wherein the ignition of the fuel is intermittent every two or four strokes of the engine, ignition within a gas turbine engine is continuous, thereby increasing the high power levels attainable by the engine.
From the combustion section, the extremely hot combustion gases are directed to the turbine section downstream of the combustion chamber. As both the turbine section and the compressor section are mounted on the same shaft assembly, rotation of the turbine blades, upon contact with the rapidly expanding and hot combustion gases, causes the shaft to which they are mounted to rotate and in turn causes the compressor blades, also mounted to the shaft, to rotate and thus complete the engine cycle. There can be additional turbine stages which are separately attached to shafts that spin fan blades or generators. The discharge of the rapidly expanding hot gases at high velocities from the turbine causes the engine to generate the aforementioned thrust needed for aircraft operation.
Typical compressors and turbines include a plurality of blades mounted on the rotor or central shaft of the engine, and a plurality of vanes on an inner engine casing, sometimes referred to as a stator. Within the compressor section, the compression ratio achievable by modern day gas turbine engines is in excess of 40:1. Such compressors can also rotate in excess of 1,000 miles/hr. and ingest in excess of 2,600 lbs/air/sec. These attributes, when combined with the continuous flow and ignition of fuel indicated above, can result in the engine generating in excess of 250,000 hp, with exhaust gases exiting the engine at speeds in excess of 1,000 miles/hr, thereby enabling commercial aircraft to cruise at the slightly less than supersonic speeds at which modern travelers have become accustomed, and military aircraft to travel at Mach speeds necessary in modern warfare.
However, in order for such engines to operate optimally, the vanes of the compressor section, those extending the engine casing, must be accurately dimensioned and mounted to ensure the incoming air is compressed as needed and does not simply flow axially through the engine. Moreover, it is often necessary for some vanes to be movable about a longitudinal axis. More specifically, such vanes are typically provided with a mounting stem or trunnion for connection to a vane arm. The vane arm is mounted so as to be rotatable and is connected to an actuator, such as a motor or other power source within the aircraft, so as to enable the vanes to rotate when the vane arm rotates.
In light of the above, one of ordinary skill in the art will readily understand that the mounting structure of the vane arm must be sufficiently robust to withstand the significant forces generated by the compressor section during not only normal operation, but when the engine experiences surge or other transients as well.
With prior art vane designs, vanes are typically mounted within vane arms using a retention device often referred to as a claw. In other words, the vane arm includes first and second appendages which wrap around the vane trunnion and insert into grooves or slots provided within the vane trunnion. A threaded fastener such as a bolt is then inserted through the vane arm and into the vane trunnion to provide additional attachment. Such a design provides a dual retention feature in that the claws are able to retain the vane in the event that the preload provided by the fastener is lost or when the entire fastener itself becomes dislodged from the vane arm. However, such a design is limited in the load conditions under which it can operate in that the claw arms tend to spread or cam away from the trunnion under high loads thereby causing the assembly to lose its capability for driving the vane to the correct angular orientation.
In another prior art design, it has therefore been known to provide a vane arm that drives the vane using an interference fit between the trunnion and the vane arm, with a loose fit being provided between another portion of the trunnion and a surge slot of the vane arm. Accordingly, when the vane arm assembly is placed under high loads and the interference fit begins to be lost due to deformation of the vane arm, the deformation causes the loose fitting area between the vane arm and trunnion to tighten, thereby providing a secondary mechanism for driving the vane under higher loads. While such a design is effective in this regard, it does not provide dual retention features in the event of fastener or fastener preload loss, and it requires relatively heavy materials at added expense.
It is also known in the prior art to provide a vane arm that has a dual retention capability to ensure that vanes of the gas turbine engine remain connected to the vane arm even under surge loads or when the fastener is lost. This is accomplished with a variable vane arm with a surge slot to facilitate rotation of the vane even when the vane is operating under surge or otherwise excessively high pressure conditions. Such a system and design is effective for dual retention, but does again require relatively heavy materials and added expense in manufacturing.
Within the context of aircraft, it is also important to understand that weight is always at a premium. The lighter the material is, the lighter the engine, and the lighter the engine, the lighter the aircraft will be. This directly translates into less fuel consumption and lower costs of operation. Designers have therefore been required to select materials which are sufficiently robust to withstand the aforementioned loads, while minimizing the weight being added to the aircraft. A still further complicating factor is that of cost. Quite often the materials which are sufficiently robust to withstand the loads encountered by the engine and still meet certain weight requirements, come at costs which make them unacceptable. Alternatively, they come at a cost which makes the overall engine cost more than is desirable.
In light of the foregoing, it can therefore be seen that a need exists for a vane arm mounting structure with improved retention capabilities even in the situation where fastener preload or the entire fastener are lost, and which can provide a mechanism by which the vane can be driven during both normal loads and surging. Moreover, it would be beneficial if such a design were to be provided wherein relatively inexpensive and light weight materials could be used in the place of materials which have traditionally been required.